I personalty find Tk easier to work with and bang out a quick working GUI.
But Wx tends to look nicer and does a far far better job adapting it's look to the windowing system its running on. So if I have the time and need to make the GUI look nice I use Wx.
Though very few of my projects start out we me having plenty of time or the need to make a nice looking GUI, so I use Tk quite a bit more.